Derivative Financial Instruments - Cash Flow Hedge Derivative Gains and Losses Recognized in AOCI and Reclassified to Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Net Gain/(Loss) Recognized in AOCI	$ 32.5	$ 7.9	$ (1.2)
Net Gain/(Loss) Reclassified from AOCI to Earnings
Net Gain/(Loss) Reclassified from AOCI to Earnings	24.2	(0.9)	(9.9)
Asset And Liability Management | Interest Rate Option Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Net Gain/(Loss) Recognized in AOCI	1.3	(3.4)	0.0
Net Gain/(Loss) Reclassified from AOCI to Earnings
Net Gain/(Loss) Reclassified from AOCI to Earnings	0.3	2.8	5.2
Net Gain/(Loss) Reclassified from AOCI to Earnings | Foreign Exchange Contracts
Net Gain/(Loss) Reclassified from AOCI to Earnings
Interest Income	19.3	6.4	0.0
Net Gain/(Loss) Reclassified from AOCI to Earnings | Asset And Liability Management | Foreign Exchange Contracts
Net Gain/(Loss) Reclassified from AOCI to Earnings
Other Operating Income	5.0	(6.4)	(8.0)
Other Operating Expense	(0.1)	(0.9)	(1.9)
Net Gain/(Loss) Reclassified from AOCI to Earnings | Asset And Liability Management | Interest Rate Option Contracts
Net Gain/(Loss) Reclassified from AOCI to Earnings
Other Operating Income	0.0	0.0	0.0
Interest Income	0.3	2.8	5.2
Other Operating Expense	$ 0.0	$ 0.0	$ 0.0